This invention relates generally to injection molding and more particularly to an improved filter assembly through which hot molten plastic material passes from the molding machine to the manifold or mold.
The hot molten plastic material or melt may contain contaminants such as wood, paper, cardboard, iron, string steel or brass which may plug the gates or interfere with operation of the valve pins, thus resulting in costly loss of production. Consequently, it is highly desirable to be able to filter the melt as it leaves the molding machine. In the past, various devices have been used for this purpose. Thin mesh screens located in front of orficed retaining plates have the disadvantage that the screen may break down and itself become a contaminant. Other devices such as radial filters have a considerable number of parts and accordingly are costly to manufacture and replace. Others have been provided with purging mechanisms, but these similarly have the disadvantage of being relatively costly and are subject to malfunction. The applicant's previous U.S. Pat. No. 4,046,359 dated Sept. 6. 1977 discloses an injection molding filter assembly which deals with these problems and while it makes substantial improvements, there are still disadvantages remaining.
A very considerable concern is to be able to pass a large volume of melt through the filter in a short period of time when injection is occurring. This is difficult in view of the restricted size of openings required in order to filter the contaminants from the melt. This problem is dealt with in U.S. Pat. No. 4,097,216 to Putkowski dated June 27, 1978 by elongating the filtering openings between the cylindrical outer body member and a fluted inner torpedo member. While also providing substantial improvements, this filter assembly similarly has ramaining disadvantages.
A major concern in injection molding is even control of the melt temperature. A drop in temperature in some areas may result in uneven flow and increasing overall temperatures to compensate for this may lead to break down of the melt. In the present case, it is important that there be no drop in melt temperature in the flow areas furthest removed from the filtering openings or undesirable dead spots may occur. This is particularly important in the slots or channels where the filtered contaminants build up. Depending upon the size of the filtering areas or slits, the size of the slots or channels may be very important in permitting sufficient melt flow without undesirable pressure drop. Another disadvantage of the previous filter assembly is that the size of the filtering area or slit cannot be changed by simply replacing a relatively inexpensive inner torpedo member by one of a different size. In order to filter different materials for different application, it is necessary that it be possible to change to a number of different filtering sizes and thus the cost of stocking the different torpedo members becomes an important factor.
Other practical difficulties in this art are the avoidance of dead spots at the inlet and outlet and accurate location of the torpedo member.